CN115667185A

CN115667185A - Silicon nitride sintered body, wear-resistant member using same, and method for producing silicon nitride sintered body

Info

Publication number: CN115667185A
Application number: CN202180038434.9A
Authority: CN
Inventors: 船木开
Original assignee: Toshiba Corp; Toshiba Materials Co Ltd
Current assignee: Toshiba Corp; Toshiba Materials Co Ltd
Priority date: 2020-05-26
Filing date: 2021-05-25
Publication date: 2023-01-31
Also published as: CN118324538A; JP2024101045A; EP4159703A4; JPWO2021241583A1; JP7528209B2; EP4159703A1; US20230093291A1; WO2021241583A1

Abstract

The present invention provides a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, wherein when a width before surface processing is set to D, a relationship between an average grain size dA and an average aspect ratio rA of the silicon nitride crystal grains in a 1 st region from an outermost surface to a depth of 0 to 0.01D and an average grain size dB and an average aspect ratio rB of the silicon nitride crystal grains in a 2 nd region inside the 1 st region satisfies a formula of 0.8 & lt dA/dB & lt 1.2 and 0.8 & lt rA/rB & lt 1.2.

Description

Silicon nitride sintered body, wear-resistant member using same, and method for producing silicon nitride sintered body

Technical Field

Embodiments relate to a silicon nitride sintered body, a wear-resistant member using the same, and a method for producing the silicon nitride sintered body.

Background

With silicon nitride (Si) ₃ N ₄ ) Since a ceramic sintered body as a main component has various characteristics such as excellent heat resistance, excellent thermal shock resistance due to a small thermal expansion coefficient, and the like, it is being applied to engine parts, steel-making machine parts, and the like as a high-temperature structural material that replaces conventional heat-resistant alloys. Further, since the cutting tool is also excellent in wear resistance, the cutting tool is also put to practical use as a rotating member or a cutting tool.

Since silicon nitride is difficult to be sintered and difficult to be uniformly sintered, various methods have been studied. In patent document 1, silicon nitride and silicon dioxide (SiO) are embedded ₂ ) The mixed powder is sintered to increase the partial pressure of the surrounding SiO gas and eliminate the weight loss, thereby obtaining a homogeneous sintered body. In patent document 2, a homogeneous sintered body is obtained by coating and sintering a mixed powder of silicon nitride, a sintering aid, and the like, thereby suppressing the evaporation of the sintering aid from the vicinity of the interface. In patent document 3, a homogeneous sintered body is obtained by controlling the ratio of the α phase and the β phase by spark plasma sintering. In patent document 4, silicon nitride and aluminum oxide (Al) are added by using ₂ O ₃ ) The heat-treated carbonaceous container is sintered to obtain a homogeneous sintered body. In patent document 5, a homogeneous sintered body is obtained by controlling the temperature reduction rate during sintering using granulated powder to which moisture is added after drying.

Documents of the prior art

Patent literature

Patent document 1: japanese patent laid-open publication No. 2002-53376

Patent document 2: japanese laid-open patent publication No. 9-77560

Patent document 3: japanese laid-open patent publication No. 9-157031

Patent document 4: japanese laid-open patent publication No. 9-235165

Patent document 5: japanese patent No. 251206

Disclosure of Invention

Problems to be solved by the invention

Silicon nitride sintered bodies have been used for various wear-resistant members such as engine parts, machine parts, bearing balls, and cutting tools. Since the silicon nitride sintered compact is particularly excellent in durability as compared with a metal member such as bearing steel (SUJ 2), long-term reliability is obtained in various wear-resistant members such as bearing balls. Thus, a long time without maintenance is also achieved.

In recent years, ceramics having excellent characteristics have been used for large bearings such as large generators, wind power generators, and aircraft engines. These large-sized parts require stricter quality characteristics than ever before, and the load applied to the silicon nitride parts used increases. However, as the ceramic member becomes larger, unevenness at the time of sintering is likely to occur, and homogeneity is not necessarily sufficient. Therefore, for example, in the production of a silicon nitride bearing ball, it is necessary to polish the surface, but a difference in the amount of machining may occur due to a difference in the microstructure between the portion near the surface and the inside.

Means for solving the problems

The silicon nitride sintered body according to the embodiment is a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, and is characterized in that, when a width before surface processing is set to D, a relationship between an average grain size dA and an average aspect ratio rA of the silicon nitride crystal grains in a 1 st region from the outermost surface to a depth of 0 to 0.01D and an average grain size dB and an average aspect ratio rB of the silicon nitride crystal grains in a 2 nd region inside the 1 st region satisfies the following expression. When the silicon nitride sintered body has a spherical or cylindrical shape, the width is the diameter of the sphere or the diameter of the circle of the cylinder.

0.8≤dA/dB≤1.2

0.8≤rA/rB≤1.2

Drawings

Fig. 1 is a view showing an example of a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment.

Fig. 2 is a view showing an example of a cross section of a silicon nitride sintered body according to an embodiment.

Detailed Description

The silicon nitride sintered body according to the embodiment, the wear-resistant member using the same, and the method for producing the silicon nitride sintered body will be described in detail below.

Fig. 1 is a view showing an example of a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment. Fig. 2 is a view showing an example of a cross section of a silicon nitride sintered body according to an embodiment.

Fig. 1 shows a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment. In fig. 1 and 2, reference numeral 1 denotes a bearing ball (sliding member), reference numeral 2 denotes a sliding surface, reference numeral 3 denotes a cross section of a silicon nitride sintered body, and reference numeral 4 denotes a sintered body surface. The abrasion resistant member using the silicon nitride sintered body is not limited to the bearing ball 1, and may be an engine member, a machine member, a bearing ball, a cutting tool, or the like. The wear-resistant member (or the silicon nitride sintered body) has a shape including a circular arc. For example, the wear-resistant member (or the silicon nitride sintered body) has a spherical shape or a cylindrical shape having a circle as an upper surface and a bottom surface. The ball includes a circular arc shape in a cross section including the center thereof. The cylinder includes a circular arc shape in a cross section parallel to the upper surface (or the bottom surface). Here, the sphere includes a regular sphere (regular sphericity = 0) and a non-regular sphere (for example, 0 < regular sphericity 0 ≦ 0.45 μm) within an error range in manufacturing the regular sphere, the cylinder includes a regular cylinder and a non-cylinder within an error range in manufacturing the cylinder, and the circle includes a perfect circle and a non-perfect circle within an error range in manufacturing the perfect circle. Hereinafter, unless otherwise specified, a case where the wear-resistant member (or the silicon nitride sintered body) has a spherical shape will be described.

For the circles having the upper and lower surfaces of the ball and the cylinder, a width, i.e., a diameter of 70mm or less is suitable. If the diameter of the wear-resistant member exceeds 70mm, unevenness in sintering tends to occur with an increase in size, and homogeneity is not necessarily sufficient. More preferably, the diameter of the circle of the top and bottom surfaces of the ball and cylinder is 60mm or less. Further, in the case of an abrasion resistant member (or a silicon nitride sintered body), that is, in the case of a ball or a cylinder having a larger circle of the upper surface and the bottom surface, for example, a diameter of 8mm or more is more effective. This is because strict quality characteristics corresponding to a large load applied to the wear-resistant member are thereby to be satisfied.

The silicon nitride sintered body according to the embodiment has silicon nitride crystal grains and a grain boundary phase. When the width of the silicon nitride sintered body before surface processing is set to D, the relationship between the average particle diameter dA and the average aspect ratio rA of the silicon nitride crystal grains in the 1 st region from the outermost surface to the depth of 0 to 0.01D and the average particle diameter dB and the average aspect ratio rB of the silicon nitride crystal grains in the 2 nd region inside the 1 st region satisfies the following formula. When the silicon nitride sintered body has a spherical or cylindrical shape, the width is the diameter of the sphere or the diameter of the circle of the cylinder.

0.8≤dA/dB≤1.2

0.8≤rA/rB≤1.2

More preferably, the relationship between the average particle diameter dA and the average aspect ratio rA and the average particle diameter dB and the average aspect ratio rB further satisfies the following equation.

0.8≤dA/dB≤0.97、1.01≤dA/dB≤1.2

0.8≤rA/rB≤0.95、1.05≤rA/rB≤1.2

A sintered body having dA/dB of about 1 and rA/rB of about 1 is preferable in terms of uniformity, but the production requires a large number of steps and a large cost.

The silicon nitride crystal grains constituting the silicon nitride sintered body achieve high strength and high toughness by growing into a needle-like shape during sintering. The shape of the needle-like crystal can be represented by the particle diameter and the aspect ratio (ratio of the long side to the short side in the rectangle). Grain growth occurs in such a manner that grain boundaries (spaces) are buried in the process of sintering silicon nitride, and the grain size and aspect ratio are increased. The strength is improved by the grain size being increased to bury the grain boundaries (spaces), but if the grain size is too large, gaps (defects) between the silicon nitride crystal grains are generated, and the strength is lowered. The aspect ratio increases with the grain growth, and the strength is improved by the complicated winding of the needle-like crystals.

When the crystal grains in the vicinity of the surface and the inside of the silicon nitride sintered body are compared, the grain size may be large in the vicinity of the surface and the aspect ratio may be small. This is because the crystal grains on the surface are brought close to a spherical shape by applying heat from the outside during sintering, gas generated from the inside of the sintered body, or the like. Particles having a large particle diameter and a small aspect ratio are less likely to be entangled with surrounding particles, and have a weak strength due to surrounding defects, and are preferentially threshed during polishing, thereby becoming a starting point for processing.

In contrast, the particle size may decrease and the aspect ratio may increase near the surface. This is because the needle-like crystals grow long and narrow depending on the sintering rate and the state of the raw material and additives. The long crystal grains are strongly entangled with surrounding particles, and are less likely to be threshed during polishing.

Thus, if the particle diameter and the aspect ratio are different between the surface and the inside, the amount of work at the time of grinding is different.

In order to eliminate the difference in processing of the entire silicon nitride sintered body during polishing, it is important to make the states of crystal grains on the surface and in the inside close to each other, and it is effective to make the grain diameters and aspect ratios of the crystal grains on the surface and in the inside close to each other.

When the average grain size dA of the silicon nitride grains in the 1 st region from the outermost surface to a depth of 0 to 0.01D and the average grain size dB of the silicon nitride grains in the 2 nd region inside the 1 st region are compared, 0.8 & lt dA/dB & lt 1.2 are set. For example, when the silicon nitride sintered body is a sphere, in a circular cross section (i.e., a cross section including a diameter) including the center of the silicon nitride sintered body, the average particle diameters dA and dB can be determined based on the silicon nitride crystal grains existing in the 1 st region and the 2 nd region each having a unit area of 20 μm × 20 μm. This is because if dA/dB is less than 0.8, the crystal grains on the surface are too small to be easily degranulated, and thus there is a possibility that processing unevenness occurs. Further, if dA/dB is greater than 1.2, the crystal grains on the surface become too large and are thus threshed, and there is also a possibility that processing unevenness will occur due to an increase in the number of processing starting points.

It is considered that the closer the range of the average particle diameter ratio is to 1.0, the less the possibility of degranulation is, and the ideal grain distribution is obtained. Therefore, a more preferred range is defined as 0.9. Ltoreq. DA/dB. Ltoreq.1.1.

When the average aspect ratio rA of silicon nitride crystal grains in the 1 st region from the outermost surface to the depth of 0-0.01D and the average aspect ratio rB of silicon nitride crystal grains in the 2 nd region inside the 1 st region are compared, 0.8 & lt rA/rB & lt 1.2 are set. For example, when the silicon nitride sintered body is a sphere, in a circular cross section including the center of the silicon nitride sintered body, the average aspect ratios rA and rB can be determined based on silicon nitride crystal grains present in the unit areas of 20 μm × 20 μm of the 1 st region and the 2 nd region, respectively, which are two-dimensional. If rA/rB is less than 0.8, the surface needle-like crystal grains become too short to be exfoliated, and the number of machining starting points increases, thereby causing uneven machining. Further, if rA/rB is larger than 1.2, the surface needle-like crystal grains are strongly entangled with each other, and the processing becomes difficult, so that processing unevenness occurs.

It is considered that the closer the range of the aspect ratio is defined to 1.0, the less the possibility of degranulation is, and the ideal grain distribution can be said. Therefore, a more preferable range is defined as 0.9. Ltoreq. RA/rB. Ltoreq.1.1.

It is preferable that the silicon nitride crystal grains having both dA and dB of 1.1 μm or more are present in 40% or more of the respective regions. This is because it is necessary to have a large number of silicon nitride crystal grains whose grain growth is sufficiently advanced to a size where the possibility of degranulation is small in order to prevent degranulation.

When the total value of elements other than Si and N in the 1 st region from the outermost surface to a depth of 0 to 0.01D is compared with the ratio pA of silicon nitride crystal grains and the total value of elements other than Si and N in the 2 nd region inside the 1 st region with the ratio pB of silicon nitride crystal grains, pA/pB is set to 0.8-1.2. For example, in a cross section including the center of the silicon nitride sintered body, the elements detected other than Si and N can be obtained by quantitative analysis of the elements per unit area in each of the two-dimensional 1 st region and the 2 nd region. This is because if pA/pB is less than 0.8, the auxiliary component is scattered from the surface, and when the sintering auxiliary component on the surface is small relative to the inside, the defects (voids) cause degranulation, and the number of processing starting points increases, thereby causing processing unevenness. Further, if pA/pB is larger than 1.2, a large amount of sintering components are present on the surface, and therefore a grain boundary phase is formed between grains, and the grain boundary phase is brittle as compared with silicon nitride crystal grains, and becomes a fracture origin and causes degranulation, which results in uneven processing.

It is considered that the closer the range of the ratio of the total value of the elements detected other than Si and N to the silicon nitride crystal grains is to 1.0, the less the possibility of degranulation is, and it can be said that the ideal distribution of the sintering aid is obtained. Therefore, a more preferred range is defined as 0.9. Ltoreq. PA/pB. Ltoreq.1.1.

The average grain size and aspect ratio of the silicon nitride crystal grains were measured as follows. First, a cross section including the center of the sphere or a circular cross section parallel to the upper surface (or bottom surface) of the cylinder is obtained. The cross section is mirror-finished to have a surface roughness Ra of 1 μm or less. When the diameter of the cross-sectional circle is D, a photograph is taken of the 1 st region and the 2 nd region inside the 1 st region from the outermost surface to 0 to 0.01D so that the 20. Mu. M.times.20 μm range can be observed with a Scanning Electron Microscope (SEM). The particle size of the silicon nitride crystal grains present in each region was measured by 50 in order of size, and the average value was determined. The reason why the average value obtained by measuring 50 particles in order of size is used as the average value of the observation plane is to prevent the particles having a small particle diameter from being infinitely included in the calculation and to make the average value vary.

The aspect ratio is determined by determining the lengths of the long and short sides of the silicon nitride particles having the measured particle diameters in each region and dividing the long side by the short side to determine the ratio. The aspect ratio is averaged.

The total value of the quantitative analysis of the elements detected other than Si and N in the silicon nitride cross section and the method of measuring the quantitative analysis of the silicon nitride crystal grains are as follows.

The mirror-finished cross section prepared by the method for measuring the average particle diameter and aspect ratio was quantitatively analyzed for silicon nitride and the added sintering aid by an Electron Probe Microanalyzer (EPMA). However, when a silicon compound is added as a sintering aid, it is difficult to distinguish it from silicon nitride, and the silicon compound is excluded from the sintering aids to be quantitatively analyzed.

Examples of the material to be added as a sintering aid for reacting to form a grain boundary phase in the sintering step include group 2 elements, group 4 elements, group 5 elements, group 6 elements, group 13 elements, group 14 elements, rare earth elements, and the like.

When the group 2 element is added, it is preferable to select any one of Be (beryllium), mg (magnesium), ca (calcium), sr (strontium), ba (barium), and Ra (radium), and it is possible to select any one or more of Be, mg, ca, and Sr. In addition, when the group 4 element is added, it is preferably selected from Ti (titanium), zr (zirconium), and Hf (hafnium), when the group 5 element is added, it is preferably selected from V (vanadium), nb (niobium), and Ta (tantalum), and when the group 6 element is added, it is preferably selected from Cr (chromium), mo (molybdenum), and W (tungsten). The group 13 element is preferably selected from B (boron) and Al (aluminum). The group 14 element is preferably selected from C (carbon) and Si (silicon). When a group 2 element component, a group 4 element component, a group 5 element component, a group 6 element component, a group 13 element component, or a group 14 element component is added as a sintering aid, it is preferable to add any of an oxide, a carbide, and a nitride.

When a rare earth element is added, it is preferable to select any one or more of Y (yttrium), la (lanthanum), ce (cerium), pr (praseodymium), nd (neodymium), pm (promethium), sm (samarium), eu (europium), gd (gadolinium), tb (terbium), dy (dysprosium), ho (holmium), er (erbium), tm (thulium), yb (ytterbium), and Lu (lutetium). When a rare earth element is added to silicon nitride during sintering, the sinterability is improved, and the aspect ratio of silicon nitride crystal grains is improved, resulting in obtaining a sintered body having very excellent strength characteristics and wear resistance.

The following describes the manufacturing method. The silicon nitride sintered body according to the embodiment is not particularly limited in the production method as long as it has the above-described configuration, but the following methods can be cited as methods for obtaining silicon nitride sintered bodies with high efficiency.

First, silicon nitride powder is prepared. The silicon nitride powder preferably contains 85wt% or more of alpha-phase silicon nitride with an oxygen content of 1 to 4wt% and has an average particle diameter of 0.8 μm or less. Since the grain boundary phase can be homogenized if the oxygen content is high, a homogeneous silicon nitride sintered body having excellent wear resistance can be obtained by growing α -phase silicon nitride powder grains into β -phase silicon nitride grains in the sintering step.

In the silicon nitride sintered body of the present invention, the surface layer and the inner surface are controlled so as to be homogeneous. To perform such control, it is effective to control the dispersion of the sintering aid. It is effective to control the amount of the sintering aid to be added and to uniformly disperse the silicon nitride powder in order to control the dispersion of the sintering aid.

The amount of the sintering aid added is preferably 2.0 to 6.0wt% of at least one of group 2 elements, group 4 elements, group 5 elements, group 6 elements, group 13 elements, group 14 elements and rare earth elements. The average particle diameter of the sintering aid powder is preferably 1.8 μm or less. The form of the sintering aid is an oxide, carbide, nitride, or the like, but the addition amount of the oxide is preferably 3.0wt% or less. This is because if an oxide sintering aid is excessively added to a raw material having a high oxygen content, the total oxygen content increases and the grain boundary phase becomes excessive.

It is effective for uniform dispersion of the silicon nitride powder and the sintering aid powder to disperse particles as objects in microscopic sizes. The crushing and mixing step by a bead mill, a ball mill, a pot mill, or the like is effective, but a bead mill is preferable for efficient production.

By constantly applying a constant stirring or vibration to the raw material compound during or after the crushing and mixing step, it is possible to prevent the silicon nitride powders from being bonded to each other, the sintering aid powders from being bonded to each other, and the silicon nitride powders and the sintering aid powders from being formed into secondary particles. The silicon nitride powder and the sintering aid powder are dispersed uniformly because most of them are primary particles.

Next, an organic auxiliary is added to the raw material mixture in which the silicon nitride powder and the sintering aid powder are mixed. For mixing the raw material mixture and the organic auxiliary, a bead mill, a ball mill or the like is used, but a bead mill is preferable for efficient production. The slurry mixed with the organic auxiliary is granulated by a spray dryer or the like, and the obtained granulated powder is formed into a desired shape. The forming process is performed by a molding press, a Cold Isostatic Press (CIP), or the like. The molding pressure is preferably 200MPa or more. The size of the compact is preferably 70mm or less in diameter in the state of a spherical sintered body. Since if the diameter of the sintered body exceeds 70mm, unevenness of sintering is liable to occur, impairing the uniformity in the vicinity of the surface and the inside.

The molded body obtained in the molding step is degreased. The degreasing step is preferably performed at a temperature in the range of 400 to 800 ℃. The degreasing step is performed in the atmosphere or in a non-oxidizing atmosphere, but it is preferable to perform the oxidation treatment at the maximum degreasing temperature. When the sintered body has a diameter of 40mm or more, the temperature is raised to 300 to 600 ℃ in a non-oxidizing atmosphere, and then the furnace is cooled to 300 to 400 ℃ and then the atmosphere or an acidic atmosphere is replaced, and the temperature is raised again to the maximum degreasing temperature. This makes it possible to control the volatilization rate of the organic auxiliary agent and to prevent the side surface of the ball or cylinder from being damaged by rapid gas volatilization.

Then, the degreased body obtained by the degreasing step is sintered at a temperature in the range of 1600 to 1900 ℃. If the sintering temperature is lower than 1600 ℃, there is a concern that the grain growth of the silicon nitride crystal grains becomes insufficient. That is, the reaction from α -phase silicon nitride to β -phase silicon nitride is insufficient, and a dense sintered body structure may not be obtained. In this case, the reliability of the material as the silicon nitride sintered body is lowered. If the sintering temperature exceeds 1900 ℃, the silicon nitride crystal grains excessively grow, and workability may be reduced. The sintering step may be performed by any one of atmospheric pressure sintering and pressure sintering. The sintering step is preferably performed in a non-oxidizing atmosphere. The non-oxidizing atmosphere may be a nitrogen atmosphere or an argon atmosphere. In addition, the atmosphere gas used is preferably circulated in a fixed amount in order to discharge the gas generated from the sintered body during sintering to the outside of the furnace.

After the sintering step, hot Isostatic Pressing (HIP) treatment of 10MPa or more is preferably performed in a non-oxidizing atmosphere. The non-oxidizing atmosphere may be a nitrogen atmosphere or an argon atmosphere. The HIP treatment temperature is preferably in the range of 1500 to 1900 ℃. By performing the HIP treatment, pores in the silicon nitride sintered body can be eliminated. If the HIP treatment pressure is less than 10MPa, the above-mentioned effects cannot be sufficiently obtained.

The silicon nitride sintered body thus produced is subjected to polishing at a necessary portion to produce a wear-resistant member. The grinding process is preferably carried out using diamond abrasive grains.

(example 1)

As the silicon nitride powder, a powder having an average particle diameter of 0.8 μm, an alphatization rate of 92% and an impurity oxygen content of 0.8wt% was used. When the total amount of the silicon nitride powder and the sintering aid was set to 100wt%, the aid powder was added so that Si was 1.0wt%, Y was 2.5wt%, and Al was 1.0wt%, and the mixture was crushed and mixed in a bead mill for 50 hours to prepare a raw material mixture.

The resin binder was mixed with the obtained raw material mixture by a bead mill to prepare a slurry. The resulting slurry is dried and sprayed by a spray dryer while being constantly stirred to produce a granulated powder. The granulated powder was press-molded at a molding pressure of 150 MPa. The press-molding was carried out by using a die having a diameter of 60mm after sintering, to obtain a spherical press-molded body. The obtained molded body was subjected to a degreasing step at 700 ℃ for 1 hour in a nitrogen atmosphere. In the degreasing step, oxidation treatment is performed by introducing air at the maximum degreasing temperature. The degreased body obtained was sintered at 1800 ℃ for 4 hours under normal pressure in a nitrogen atmosphere. The nitrogen flow rate at the maximum sintering temperature in the atmospheric sintering was set at 30L/min. The internal volume of the sintering furnace used for sintering was about 0.9m ³ (900L). The obtained sintered body was subjected to HIP treatment at 1600 ℃ C.. Times.20 MPa.times.2 hours.

The silicon nitride sintered body of the sphere was cut and mirror-polished at an arbitrary cross section of a circle including the center of the silicon nitride sintered body, and then magnified photographs (SEM photographs) were taken from the surface to the vicinity of 0.3mm (0.005D) and from the surface to the vicinity of 1.8mm (0.03D). As a result of setting a unit area of 20 μm × 20 μm from the magnified photograph and obtaining the average particle size and the aspect ratio for 50 particles in order of particle size, the average particle size (dA) of a cross section from the surface to 0.3mm was 1.16 μm, the aspect ratio (rA) was 2.0, the average particle size (dB) from the surface to 1.8mm was 1.05 μm, and the aspect ratio (rB) was 2.1. Therefore, dA/dB was 1.10 and rA/rB was 0.95. Then, the ratio of the average particle diameter (dA and dB) was measured from each magnified photograph to be 1.1 μm or more, and as a result, the ratio was 49% at a position from the surface to 0.3mm and 47% at a position from the surface to 1.8 mm.

In addition, quantitative analysis of Si, al and Y was carried out by EPMA at the same positions as observed by SEM. The ratio (pA) obtained by dividing the total of the quantitative analysis values of Al and Y elements, which are elements detected at a position of 0.3mm from the surface except Si and N, by the quantitative analysis value of Si was obtained, and the result was 0.037. Also the ratio (pB) from the surface to the position of 1.8mm was 0.036. Thus, pA/pB was 1.03.

After removing projections and the like on the surface by rough machining, the sintered body produced under the same conditions was machined by a polishing machine under the condition of medium finish machining (3 μm abrasive grain) for 10 hours and under the condition of finish machining (0.25 μm abrasive grain) for 4 hours. The finished spheres were measured for diameter variation (difference between maximum and minimum values), sphericity, and surface roughness (Ra) by setting an arbitrary circumferential direction, and were 0.28. Mu.m, 0.24. Mu.m, and 0.027. Mu.m, respectively.

Then, the Hardness (HV) and the three-point bending strength (σ f) of each silicon nitride sintered body were measured, and as a result, the hardness was 1480 and the bending strength was 880MPa. The three-point bending strength measurement sample (silicon nitride sintered body) was processed to have a size of 3mm × 4mm × 50mm, and measured by a method according to JIS-R-1601.

(examples 1 to 6 and comparative examples 1 to 4)

Test pieces of the silicon nitride sintered body were produced under other production conditions based on example 1. Table 1 shows examples (1 to 6) and comparative examples (1 to 4) of the type and amount of the sintering aid, the method of crushing and mixing the aid (mixing time), the method of mixing the organic aid (mixing time), the degreasing conditions (degreasing temperature and presence/absence of oxidation treatment), and the sintering conditions (sintering temperature-sintering time-gas flow rate). In addition, in the comparative example, stirring was not performed until spray-drying was performed using a spray dryer. The other conditions were the same as in example 1. The amount of the sintering aid added is a ratio in which the total amount of the silicon nitride powder and the sintering aid is 100 wt%.

TABLE 1

Table 2 shows the average particle size dA of the silicon nitride crystal grains of the circular arbitrary cross section in the 1 st region from the outermost surface to the depth of 0 to 0.01D, the ratios (dA/dB) of the average particle size dB and dA and dB in the 2 nd region inside the 1 st region, the average aspect ratio rA of the silicon nitride crystal grains of the circular arbitrary cross section in the 1 st region, and the ratios (rA/rB) of the average aspect ratios rB, rA and rB in the 2 nd region in examples 1 to 6 and comparative examples 1 to 4. The silicon nitride sintered bodies described in examples 1 to 6 and comparative examples 1 to 4 had a diameter of 8mm to 70 mm.

TABLE 2

Table 3 shows the ratio (%) of the area occupied by the region having an average particle diameter dA of 1.1 μm or more in the circular arbitrary cross section in the 1 st region from the outermost surface to the depth of 0 to 0.01D, the ratio (%) of the area occupied by the region having an average particle diameter dB of 1.1 μm or more in the 2 nd region inside the 1 st region, the ratio pA of the total value of elements other than Si and N and the silicon nitride particle, the ratio pB, pA and pB (pA/pB) of the total value of elements other than Si and N and the silicon nitride particle, which are detected by quantitative analysis of the elements per unit area in the circular arbitrary cross section in the 1 st region, in examples 1 to 6 and comparative examples 1 to 4.

TABLE 3

Table 4 shows that the finished spheres of examples 1 to 6 and comparative examples 1 to 4 have arbitrary circumferential difference (difference between maximum value and minimum value), positive sphericity, surface roughness (Ra), hardness (HV), and three-point bending strength (σ f).

TABLE 4

The silicon nitride sintered bodies according to the examples and comparative examples had high values of 1400 or more in hardness and 760MPa or more in three-point bending strength.

The silicon nitride sintered bodies according to examples 1 to 6 were all 0.5 μm or less in diameter, 0.45 μm or less in spherical sphericity and 0.04 μm or less in surface roughness (Ra).

In contrast, in comparative examples 1 to 4, the diameters were respectively different from 0.71 to 1.01. Mu.m, the sphericity was 0.76 to 1.10 μm, and the surface roughness (Ra) was 0.05 to 0.97. Mu.m, although the same processing conditions were applied, and they were all relatively large as compared with the examples.

From these experimental results, it is considered that the examples are extremely excellent in the surface workability, can suppress the difference in the surface workability and the inner workability, and can suppress the variation in the processing quality and the dimension in the batch processing.

Several embodiments of the present invention have been described above, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalent scope thereof. The above embodiments can be combined with each other.

Claims

1. A silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, wherein,

when the width before surface processing is set as D, the relationship between the average particle diameter dA and the average aspect ratio rA of the silicon nitride crystal grains in the 1 st region from the outermost surface to the depth of 0-0.01D and the average particle diameter dB and the average aspect ratio rB of the silicon nitride crystal grains in the 2 nd region inside the 1 st region satisfies the following formula,

0.8≤dA/dB≤1.2

0.8≤rA/rB≤1.2。

2. the silicon nitride sintered body according to claim 1, wherein both the average particle diameter dA and the average particle diameter dB of the silicon nitride sintered body are 1.1 μm or more.

3. The silicon nitride sintered body according to claim 1 or 2, wherein 40% or more of the silicon nitride crystal grains are present in both the 1 st region and the 2 nd region.

4. The silicon nitride sintered body according to any one of claims 1 to 3, wherein a relationship between a total value of elements other than Si and N and a ratio pA of the silicon nitride crystal grains in the 1 st region and a total value of elements other than Si and N and a ratio pB of the silicon nitride crystal grains in the 2 nd region satisfies 0.8. Ltoreq. PA/pB. Ltoreq.1.2.

5. The silicon nitride sintered body according to claim 4, wherein the elements other than Si and N to be detected are determined by quantitative analysis of the elements per unit area.

6. The silicon nitride sintered body according to any one of claims 1 to 5,

the relationship between the average particle diameter dA and the average aspect ratio rA, and the relationship between the average particle diameter dB and the average aspect ratio rB also satisfy the following formula,

0.8≤dA/dB≤0.97、1.01≤dA/dB≤1.2

0.8≤rA/rB≤0.95、1.05≤rA/rB≤1.2。

7. the silicon nitride sintered body according to any one of claims 1 to 6, wherein the average particle diameter dA, the average aspect ratio rA, the average particle diameter dB, and the average aspect ratio rB are determined based on the silicon nitride crystal grains present in each of the 1 st region and the 2 nd region at a unit area of 20 μm × 20 μm.

8. The silicon nitride sintered body according to any one of claims 1 to 7, which has a spherical shape or a cylindrical shape having a circle as an upper surface and a bottom surface.

9. The silicon nitride sintered body according to claim 8, wherein the diameter of the sphere or the circle as the width is 8mm or more and 70mm or less.

10. The silicon nitride sintered body according to claim 8, wherein the diameter of the sphere or the circle having the width is 8mm or more and 60mm or less.

11. A wear-resistant member, characterized by using the silicon nitride sintered body according to any one of claims 1 to 10.

12. A method for producing a silicon nitride sintered body according to any one of claims 1 to 10, wherein,

the method comprises a molding step of molding a granulated powder obtained by granulating a raw material mixture in which a silicon nitride powder and a sintering aid powder are mixed, at a pressure of 200MPa or more.